Recent personal investigations into the pathogenesis of the deleterious effects of sunlight on skin (viz. sunburn, premature aging, pre-malignant and malignant lesions) have revealed that important etiologic factors have been either overlooked or disregarded by investigators in this field, namely long ultraviolet (LUV) and visible (VIS) light. Contrary to commonly held beliefs, LUV is neither harmless nor helpful to human skin. In fact, LUV was found to severely enhance the short UV sunburn response "photo-augmentation phenomenon" and to be a potential carcinogen. Natural sunlight, xenon solar-simulating and monochromatic light will be utilized to test human volunteers and certain other mammals to determine the following: (1) the photoaugmentative potentialities of different LUV and VIS wavelengths alone and in various combinations (i.e., action spectroscopy), (2) the duration of susceptibility to photoaugmentation following exposure to short UV, LUV, or VIS light of different doses when given in different sequences, (3) histological, electron microscopic, biochemical and physiological changes that occur following varying single and repeated exposures to LUV, VIS, and short UV given alone and in combinations, and (4) whether the precancerous changes that develop in skin after repeated LUV exposure are the results of mainly epidermal or underlying stromal changes. Knowledge from these studies will be utilized to: (1) elucidate the mechanism of photoaugmentation and the role of LUV in the induction of actinic cancer, (2) develop guidelines for prevention of hazardous long-wavelength light exposure, (3) develop methods, materials, and/or agents to protect against photoaugmentation and its sequelae, (4) explore the need to develop new guidelines for treatment of skin cancer.